The spring-loaded gas pressure regulator R 51 has the function of keeping the outlet pressure of a gaseous medium constant within permissible liwith values, independently of the effect of interferences, such as changes in the inlet pressure and/or in the gas flow, in the connected regulating line on the outlet side. The regulator is composed of the actuator housing and “diaphragm assembly plus actuator” functional unit. The valve seat model is pre-pressure-compensated. The gas flows through the actuator housing in the direction of the arrow. The internal or external measurement line port is used for passing the outlet pressure to be regulated to the bottom of the diaphragm comparator of the diaphragm assembly. It compares the actual value with the command variable preset by the force of the setpoint spring. The setpoint required in each case is set via the setting screw. Any deviation from the setpoint is transwithted by the valve rod to the actuator, which is adjusted such that the actual value is adjusted to the setpoint. In case of zero flow, the actuator will close tight, causing the closing pressure to be established.